This invention relates to a system for simultaneously coding and deconding a plurality of signals and more particularly to a system in which a voice signal is divided into a plurality of bands so as to efficiently code and decode the voice signals.
Adaptive quantizing is widely used to efficiently code and decode signals, especially when the level of the signal varies greatly.
According to one example of adaptive quantization, an automatic gain control (AGC) circuit is placed before a quantizer. The AGC circuit decreases its gain for large signal amplitude and increases its gain for small signal amplitude of the original. As a result, the level variation of the input to the quantizer is suppressed even when the input signal level varies greatly. Consequently, the quantization distortion can be minmized, even with a small number of quantization bits.
According to another example of adaptive quantization, the spacing of quantization (step size) of the quantizer is adaptively varied, thereby attaining substantially the same effects as the former method based on the AGC circuit. In the adaptive quantization utilizing the AGC circuit, the step size or quantization spacing is related to the gain of the AGC circuit as will be described later. The term "quantization bit number" is used herein to mean the number of levels spaced from each other by a given step size. Taking the quantization bit number being 4 and the quantization spacing being d, for instance, a scheme may be conceivable wherein +1 to +4 levels and -1 to -4 levels with spacing d between adjacent levels are set for quantization with respect to a reference of zero level.
Although in the foregoing, a case wherein a single signal is quantized has been considered, there are many cases wherein two types of signals are quantized, as in stereo broad-casting, or when the bandwidth of a single signal is divided into two. In such cases, quantization with small distortion can be realized by assigning the quantization bit number in accordance with the level difference between two signals. One example of such an encoding method is sub-band coding (SBC). According to this coding method, a signal is divided into a number of bands (that is sub-bands) and then the sub-bands are coded separately. This method is detailed in U.S. Pat. No. 4,142,071 to Alain Croisier et al entitled "Quantizing Process with Dynamic Allocation of the Available Bit Resources and Device for Implementing said Process" and will not be described herein. In conclusion, according to this method, it is necessary to transmit to the decoding side information regarding the quantization bit number assigned to each sub-band at the time of coding.
The method of varying the number of assigned bits according to the amplitude of signals is termed "adaptive bit assignment". The above-referenced U.S. Patent describes that when the adaptive bit assignment is made, transmission quality can be improved but hardware becomes complicated and the adaptive bit assignment is not always the best method.
As will be described later in detail with reference to the accompanying drawing, use of the adaptive bit assignment can decrease quantization distortion but it requires transmission of information regarding bit assignment and complicated circuits.
Moreover, since it is necessary for a bit assignment circuit to suppress the greatly varying input signals for quantization bit number reduction purposes, the signal in a quantizer must be so delayed as to maintain synchronization between the quantizer and the bit assignment circuit.